The present invention relates in general to fold-back type current limiting circuits, and in particular to overcurrent protection circuits for AC load-energizing semiconductor switches such as target-actuated, line-operated, solid state proximity switches.
It is generally accepted that in many applications solid state devices are superior replacements for relays, mechanical switches, and other electromechanical devices because of their inherently high reliability and speed due to the lack of moving parts. It is also well known that such solid state devices are more prone to failure under overvoltage and/or overcurrent conditions than their electromechanical counterparts.
A particularly troublesome area of solid state device failure is in AC line-operated (110-220 VAC) semiconductor switch applications, such as the application of series-inserted, solid state proximity switches for energizing AC loads actuated by a target tripping the proximity switches, the target being, for example, an assembly line product moving past work stations of a manufacturing plant.
Wiring for such line-operated semiconductor switches may cover large distances in the manufacturing plant. Mechanical abuse or incorrect connection of the switch wiring may cause a short circuit condition which, if left uncompensated, will destroy the semiconductor switch when actuated by the target into a conducting condition.
Fold-back type overcurrent protection circuits for insulating devices from the effects of short circuit conditions are known. U.S. Pat. No. 3,638,102 illustrates such an overload protection circuit, usually connected in series with the device to be protected, wherein current through the protection circuit is limited to a preset maximum value under overload conditions, the protection circuit being at a high impedance state. Upon clearing of the overload fault, the illustrated fold-back protection circuit automatically returns to a normal low impedance operating state to permit normal current flow. In many line-operated, proximity switch applications, it is not desirable for safety reasons to restore full power to the load upon clearing of a short circuit fault.
It is an object of the present invention to provide a resettable, lockout type short circuit protector having particular application to the protection of line-operated, solid-state proximity switches wherein short circuit fault clearing would not result in immediate re-energization of the load.